When you set workflow on a standard value of a template At the point you create a new content item based on that template, this item will get the workflow applied, however the content that was made before the standard value was introduced do not inherit this setting.

1) Standard value will push through if the field has not been set before.
2) If there was a default workflow for the item previously, the new workflow will not get push through.
3) You can go to version ribbon and hit the "reset fields" button to get this to update.
4) Look into Sitecore Powershell Module to see how you could write a script to automate this update on content items already existing.